Although Cu—Zn alloy has lower spring properties compared to phosphor bronze, beryllium copper, and Corson alloy etc., it is cheaper and is thus widely used as electric contact materials such as a connector, a terminal, a relay, and a switch. Representative Cu—Zn alloy is brass, and alloys such as C2600 and C2680 are specified in JIS H3100. When using Cu—Zn alloy for an electric contact material, it is often applied Sn plating to obtain stably low contact resistance. Taking advantage of superior solderability, corrosion resistance, and electrical connectability of Sn, Sn plating strip of Cu—Zn alloy is used in large amounts in a terminal for wire harness of automotive electrical equipments, a terminal for printed circuit board (PBC), and electrical and electronic parts of a connector contact for household appliances etc.
Typically, when a reflow Sn plating strip of copper alloy is kept at an elevated temperature for a long period of time, a phenomenon in which the plating layer is peeled off from the base material occurs (hereinafter referred to as thermal peeling). When Zn is added to the copper alloy, thermal peeling property will be improved. Accordingly, the thermal peel resistance of Cu—Zn alloy is relatively good.
The above Sn plating strip of Cu—Zn alloy is manufactured in the steps of degreasing and pickling, and then formation of an undercoat layer by electroplating, followed by formation of an Sn plating layer by electroplating, and finally application of reflow treatment to melt the Sn plating layer.
A common undercoat for the Cu—Zn alloy Sn plating strip is a Cu undercoat. For applications that require thermal resistance, a Cu/Ni bilayer undercoat may be applied. As used herein, a Cu/Ni bilayer undercoat is a plating in which electroplating is performed in the order of an Ni undercoat, a Cu undercoat, and an Sn plating, and then reflow treatment is applied. The constitution of the plating coating layer after reflow treatment will be, from the surface, the Sn phase, the Cu—Sn phase, the Ni phase, and then the base material.
Details on this technology are disclosed in the following patent application documents 1-3 (Japanese Published Unexamined Application 6-196349, Japanese Published Unexamined Application 2003-293187, and Japanese Published Unexamined Application 2004-68026).